Omar Gonzalez's 94th-minute goal at Stubhub Center gave LA Galaxy back-to-back wins in thrilling fashion, his on-target poke from a corner deflecting in off the knee of Sporting KC substitute Bernardo Anor.
Baggio Husidic had put Galaxy ahead in the ninth minute, but Krisztian Nemeth's equalised 13 minutes later.
Galaxy moved second in the Western Conference, but finished the matchday in third after Dallas' late 3-2 victory over Toronto.
Castillo scored in the first minute, and doubled his tally in the 10th before Blas Perez made it 3-0 in the 27th minute.
The stadium lost power in the 42nd minute to cause the lengthy delay, and the match eventually re-started just before midnight local time.
Sebastian Giovinco set up a tense finale with a late double, but Toronto were unable to salvage a point.
Lamar Neagle scored twice for the Sounders, either side of Gabriel Torres' strike for Colorado, before Obafemi Martins controlled a long ball with a swift move on the turn between two Rapids defenders to fire home and seal a 3-1 win..
Columbus Crew won for the first time in four outings, seeing off 10-man Orlando City 3-0.
Rafael Ramos was sent off for the new franchise in the 34th minute, just moments after Crew opened the scoring through Federico Higuain.
Darren Mattocks' 80th-minute goal sent Vancouver Whitecaps to a 1-0 win at Real Salt Lake, who finished with nine men.
